This invention relates to electronic surveillance systems, and more in particular to an improved field generation and reception arrangement for use in such systems.
Electronic surveillance systems of the type to which the present invention is directed, are generally employed to detect the presence of a magnetic marker in a magnetic field. Such systems thus include a device for generating a magnetic field, and a receiver for detecting variations in the field resulting from passing of a marker, generally carried by an article, through the field.
Such a system is disclosed, for example, in U.S. Pat. No. 4,71C,752, Cordery. In the system disclosed in this reference, the ability of the system to detect the presence of a marker, in the presence of noise, is enhanced by forming the magnetic field of more than one frequency component. The ferromagnetic marker effects the modulation of the higher frequency component with the lower frequency component to produce output pulses at the frequency of the higher frequency field component and its harmonics that are modulated by the lower frequency component and its harmonics. The sidebands of the pulses output from the marker are readily distinguished from noise, to increase the ability of the system to distinguish the presence or absence of a marker in the field. Such a system thus enables detection of smaller tag signals and/or increased spacing between the signal generator and the receiver.
A further electronic surveillance system for generating a magnetic field and receiving signals therefrom is disclosed in U.S. Pat. No. 4,872,018. In this patent, the transmitter and receiver are each provided with an antenna having two pairs of twisted loops. The twists are provided in the transmitting antenna to confine the transmitted signal to an area close to the transmitter and reduce the amount of signal outside the immediate vicinity of the transmitting antenna.
In systems of the above type, a magnetic marker "switches", to provide a detectable "output", when the externally applied field passes the coercive field of the ferromagnetic marker. If the marker is biased, then the phase of the externally applied field at which the marker switches is changed.
Soft magnetic markers for electronic surveillance systems are disclosed, for example, in U.S. Pat. Nos. 3,631,442; 3,747,086; 3,665,449 and 3,983,552 and French Patent No. 763,681.